Even in freezing temperatures, an ice cube can form a nanometers-thick sheen of water on its surface. This peculiar behavior, often called surface melting or pre-melting, was first proposed by Michael Faraday in 1846 as an ever-present precursor to actual melting.
But this past February, a team of Japanese researchers published a study offering a different perspective on the phenomenon: instead of melting from the outside in, the researchers showed that this so-called quasi-liquid layer (QLL) condenses on the ice surface, like morning dew on grass. Now, that same team has presented evidence that takes the phenomenon further from the realm of melting, showing that QLLs also form as ice sublimates into vapor—not water. They’ve also proposed a theoretical model based on surface tension to explain this unexpected behavior, which could help scientists understand how ice and snow interact with their surroundings—both in clouds and on the ground.
“The new paper, I think, is very valuable,” says surface chemist Thorsten Bartels-Rausch of the Paul Scherrer Institute in Villigen, Switzerland. He was not involved in the work, but says it has “already raised a lot of interest” in the community.
The study, published online in PNAS on October 17, comes from the Institute of Low Temperature Science at Hokkaido University in northern Japan. The team studies these ultra-thin films with an advanced optical microscope developed by team member Gen Sazaki in collaboration with Olympus, the camera and microscope maker. The device, which combines two advanced microscopy techniques—laser confocal microscopy and differential interference contrast microscopy—can distinguish differences in height as small as a tenth of a nanometer.
The team used this apparatus in the February study which revealed that the longstanding term “surface melting” was a complete misnomer: instead, the QLL was condensing onto the surface from the supersaturated air around it. In the new work, they show that QLLs also form when ice sublimates into water vapor. In fact, contrary to Faraday’s original idea of an ever-present film, their images suggest that QLLs can only form when the ice is growing or shrinking—not when it’s in equilibrium.
This added to a growing mystery: in 2012, the team reported that QLLs don’t just appear as a uniform thin film, but sometimes as droplets—and sometimes droplets that form on top of the film, resembling eggs fried sunny-side up. At the time, the team attributed this strange behavior to two separate thermodynamic phases, and they went so far as to label them alpha and beta, unable to reconcile them.
But when experimental physicist Ken-ichiro Murata joined the lab in 2014, he immediately recognized the sunny-side up shape as a long-theorized phenomenon in the physics of wetting, called pseudo-partial wetting. He’d written a paper on the topic before. “This was very lucky for me,” he says, referring to his observation of the shape. The physics of how water coats a surface (wetting) or breaks apart into droplets (dewetting) are governed by surface tension. Based on these principles, Murata quickly sketched a thermodynamic model in which QLLs are a transitional phase between ice sublimating to vapor and vice versa.
In addition, the study’s findings that QLLs can be found both when snow and ice grows and shrinks through sublimation is a “very important key factor if we want to understand the environment,” says Bartels-Rausch. As an example, he points to freshly fallen snow, which skiers and snowboarders know becomes harder as ice crystals grow. “There are really huge fluxes of water vapor,” he says. And if the new paper is right, the behavior of QLLs is a key part of understanding those processes. “It will be very interesting for the whole environmental community.”